Two-stage electrostatic separation of particulate material

ABSTRACT

Separation of potassium oxide containing salts into various size fractions by passing a mass containing particulate and powder fractions between two electrodes of an electrostatic precipitator; maintaining the voltage across the electrodes such that the mass separates into portions, that part adheres to the positive electrode, part stays adjacent the positive electrode, part stays adjacent the negative electrode, and part passes intermediate the two electrodes; and recovering the individual parts separate from each other.

O United States Patent H 1 3,581,886

1 Inventors Arno singegvald [56] References Cited Kassel-Wil Guenter Fricke, Roeddense Ueber Lehrte; 3 2 I 7 87 UNITED STATES PATENTS Heinz Erbe Diekho'zen Ueber Hildesheim 6 l 1/1965 Autenr eth 209/9 a of Germany 3,477,566 1 H1969 Autenrieth et al 209/9 [21 Appl. No. 839,367 FOREIGN PATENTS [22] Filed July 7, 199679 6,336 1913 Great Britain [45] Patented June 1,1 1 [73] Assignee Wintershall Aktiengesellschaft OTHER REFERENCES Kass, Germany Autenrleth, German Application 1 154052 printed Sept. 12, 321 Priority Nov. 9, 1965 963(KL156) [33] Germany Primary Examiner-Frank W. Lutter [3 l W40257 Assistant ExaminerRobert Halper Continuation-impart of application Ser. No. An0rneyMichael S. Striker 592,863, Nov. 8, 1966, now abandoned.

541 TIC SEPARATION OF ABSTRrKCT: Separation of potassium oxide containing salts 12 Claims 1 Drawin Fi into various size fractions by passing a mass containing parg g ticulate and powder fractions between two electrodes of an 1152] 11.8. C1 209/9, electrostatic precipitator; maintaining the voltage across the 209/127 electrodes such that the mass separates into portions, that part [51] Int. Cl B031) 1/04, adheres to the positive electrode, part stays adjacent the posi- B03c 7/08 tive electrode, part stays adjacent the negative electrode, and 1,130] Field of Search 209/ 127- part passes intermediate the two electrodes; and recovering l 3 1 9 the individual parts separate from each other.

wo-suc ELECTROSTATIC SEPARATION or ART cuLA EM TEmAL CROSS-REEERENCES TO RELATED APPLICATIONS This is a continuation-in-part of our copendinglapplication, Ser. No. 592,863 now abandoned, filed on Nov. 8,1966, under the title Method and Apparatus for Electrostatic Separation of Particulate Material. I

BACKGROUND OF THE INVENTION The presentinvention relates to a method for the electrostaticseparation of particulate material and, more specifically,

, the invention is concernedwith, a; method, for the electrostatic treatment of crude potassium saltsin. particulate formand including a fraction of very small, dustlike particles.

Theseparation of. sylvitefrom. dust-containing crude potassium salts is connected with very-considerable difficulties which itwas not possible up tonow toovercome in a satisfac tory manner. These difficultiesarise primarily due tothe fact that the dustlikefraction. of the crudepotassium-salts will be unselectively charged in an electrostatic field and, consequently, the dustlike particles .which will be collected together. with thecoarser particles of higher potassium contentand since these dustlike particles atleast partially have a relatively low potassium content, theirpresence will reduce the .K Oconcentration in the separated particulate fraction of higher K content. The dustlike fraction is. primarily selectively charged. It appears to be unselectively charged because its constituent particles willreadily agglomerate, in which condition the selectivecharge enchange isnullified with resultant adverse influencingof thek O content of the concentrates and the residues. Thisoccurs particularly, if the crude material includes dustlike fraction in a quantity of 25 percent or. more. If

a crude .salt is electrostatically processedin accordance with the approach knownfrom the prior art and without prior separation of the dustlike,fraction,;a large quantity of the dustlike fraction will enterthe concentration stage, together with tlie preconcentrate, and will be returned from the concentration stage to the :preconcentration stage together with the middlings. This quantity of dustlike fraction, that is the quantity contained inthe middlings, will inevitably causethe maximum pennissiblequantityof up .to 25 percent-in-the finished product to be exceeded Throughout the description ,and claims thepotassium-content will be referred .to as K 0 as an indication of the equivalent amount of the potassium salts which are actually present.

A further difiiculty in the concentration of sylvite from dust-containing particulate crude potassium salts is caused by the firm adherenceof the dustlike particlesat the, electrodes of the electrostatic separator, whereby again the electrostatic separation of the, coarser constituents of the particulate mass of crudepotassium salts into fractions of higher and lower K 0 content will be impaired.

In view of the foregoing, it has been proposed to separate dustlike particlesfrom the particulate mass of crude potassium salts by mechanical means such as screening, prior to the electrostatic.dressing of the particulate mass. However, such prescreening requires aseparateinstallation and is a relatively expensive procedure and, furthermore, results inthe separation of the dustlike particles in a form inwhich same are substantially without commercial value so that the thus separated mass of dustlike particles has to-be further processed, for instance concentrated by. dissolutionin hotwaterfollowedby crystallization. I

The directelectrostatic separation of a mass consisting essentially only of dustlike particles into-fractions of higher and lower K 0. content is notpossible in a technically satisfactory manner, due to the lack of selectivity of the dustlike particles with respect to the acceptance ofelectrostatic charges by-the same.

By'producing a concentrate in conventional manner, by electrostatic separation which may be carried out as a single step or as a two-step electrostatic separation, it is possible, for

- instance, to separate hard salt into the following fractions:

Concentrate,

Pie-concentrate, Residue, Percent K20 Percent K20 Percent K20 Pro-concentrate, Concentrate, Residue, Percent K20 Percent K20 Percent K20 The above examples will serve to show that it is not possible by using the above-described conventional processes to work in a technically satisfactory manner with crude potassium salts which contain a significant proportion of dustlike particles, particularly hard salt.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the above-discussed difficulties and disadvantages.

It is a further object of the present invention to provide a method which will permit the working-up of crude potassium salts containing a-significant proportion of dustlike particles so as to obtain a separated concentrated fraction of relatively high K 0 content and a residue of relatively very low K O content.

It is a further object of the present invention to provide a method for the electrostatic separation of dust-containing crude potassium salts into fractions of higher and lower K 0 concentration which can be carried out in a simple and particularly effective manner.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention contemplates a method of electrostatically separating a particulate mass of crude potassium salts, including a dustlike fraction and coarser particles of varying K 0 content, into the dustlike fraction and into a plurality of fractions of progressively lower K 0 concentration and substantially free of dustlike particles, comprising the steps of passing a stream of the mass through an electrostatic field formed between electrodes of opposite polarity, the electrodes, respectively, exerting different degrees of attraction with respect to the dustlike particles and thecoarser particles of higher and lower K 0 content of the stream, so as to separate the mass into the dustlike fraction and into a stream of coarser particles having a progressively lower K 0 concentration in the direction from one of theelectrodes to the other, and separately recovering the dustlike fraction, and the portions of the stream of highest, lowest and intermediate K 0 concentration.

The present invention also provides in a device for the electrostaticseparation of particulate material into fractions of particles having, respectively, different electrostatic characteristics with respect to attraction of the same by electrodes of opposite polarity, in combination, a pair of elongated electrode means including electrodes of opposite polarity, respectively, the electrode means being arranged spaced from each other andforming between themselves a substantially vertical path for the downward passage of a stream of particulate material therethrough. at least one of the electrode means including endless belt means rotating about the electrode of the one electrode means and in part forming the path so that particles which are strongly attracted by the one electrode will be caused to adhere to the moving belt means, brush means opcratively associated with the moving belt means at a portion thereof spaced from the path for separating adhering particles from the belt means, and guide means located in the vicinity of the lower end of the path for preventing contact between the downwardly passing stream and the separated particles, and for recovering of the latter.

According to the present invention, it is possible to work up dustlike particles-containing crude potassium salts in a manner which permits separation of the dustlike fraction in the fomi of a commercially valuable product containing about 40 percent K 0.

Whereas according to the conventional methods of separating and concentrating crude potassium salts in an electrostatic separator three fractions are produced, namely a preconcentrate, middlings and residue, the present invention provides for the additional separation of a further concentrated fraction, namely a dustlike fraction consisting of very small particles, generally having a size of less than 0.15 mm. and containing about 40 percent K 0.

This additional separation of the dustlike fraction having a particle size of below about 0.1 or 1.15 mm. is carried out simultaneously with the separation of the coarser particles of the crude salt into preconcentrate, middlings and residue. Thereby the further advantage is achieved that the separated fractions of coarser particles will be substantially free of dustlike particles.

Preferably, the method of the present invention is carried out in an electrostatic separating device in which the crude salt particles pass under the force of gravity downwardly through an electrostatic field produced by two spaced electrodes which define the path through which the stream of particulate potassium salt moves in a downward direction. The crude potassium salt is preferably preconditioned in conventional manner prior to being introduced into the electrostatic separator, for instance by treatment with straight-chain lower fatty acids. According to the present invention, means are provided for dislodging dustlike particles which under the force of the electrostatic field attach themselves to an electrode, and for separately recovering the thus-dislodged dustlike particles.

It has been found, surprisingly, that the dustlike fraction will adhere to the electrodes at a certain specifically defined field strength, while the coarser particulate fraction will freely travel over the electrodes. This makes it possible to brush the adhering dustlike fraction off the electrodes and to separate it from the final product through the use of a separating trap which is adjustable for particle separation.

Only after this was understood did it become possible to develop the present approach to the processing in electrostatic separators, particularly free-fall separators, of crude potassium salts containing dustlike fraction. For example, to separate the dustlike fraction below approximately 0.15 mm. grain size when raw salts are conditioned with a l5 percent water solution of Na-salts of fatty acids, a field strength of 3.5-4.5 kv./cm., preferably 4.0-4.5 kv./cm., is necessary.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a schematic elevational view in the nature of flow diagram, of a two-step electrostatic separation process according to the present invention. 1 represents the dust concentrate, 2 are the resultant middlings introduced into the second separator, 3 the residue, 4 the coarse preconcentrate, 5 the middlings from the second separator to be recycled, 6 represents the endless belt electrode, and 7 the brush means DESCRIPTION OF THE PREFERRED EMBODIMENTS By proceeding in accordance with the present invention of electrostatic separations it is possible, for instance, from a hard salt containing 19.2 percent sylvite, 35.4 percent kieserite, 0.9 percent carnallite, 0.34 percent anhydrite, 1.64 percent insolubles and 0.23 percent moisture, and having the following grain size distribution:

The sieve analyses of the products obtained are as follows.

Dust-free Dust preooncentraction trate Concentrate Mllllmeters:

Greater than 1.0 0. 4 0. 1 0.751.0 3.5 1.2 18. 3 16. 0 1.3 31. 7 43. 0 3. 3 20. 2 20. 3 21. 0 20. 6 16. 9 27. 0 4. 1 2. 3 47. 4 1. 2 0. 2

It is thus possible to obtain from about 20 tons of the abovedescribed crude potassium salt in the first electrostatic separation about 2 tons of a dustlike concentrate 1 of commercially marketable quality. The dust-concentrate 1 may be sold and used as such, for instance in the production of mixed or potassium fertilizers, or it may be further .worked up, in a manner known per se, to form a granulated and commercially valuable product.

It will also be seen from the example given above that, by proceeding in accordance with the present invention, it is possible in the postseparation to further work up the concentrate 4 obtained in the first separation, so as to arrive in the postseparation a concentrate which has a K 0 content which is by 10 percent higher than that which could be obtained without the separation of dustlike particles in the prior art electrostatic separation processes.

The possibility, according to the present invention, to obtain in a simple and effective manner a commercial product having a K 0 content of between 58 and 60 percent is of very great practical significance.

Thus, the method of the present invention permits to obtain from a crude potassium salt including dustlike particles between two-thirds and three-fourths of the potassium, calculated as K 0 in the form of a concentrated product containing between 58 and 60 percent K 0, and between one-third and one-fourth as a product containing between 38 and 40 percent K 0.

It is also possible to utilize the dust concentrate 1 which is obtained in the separation of hard salt and which contains between 38 and 40 percent K and about percent MgO for producing a granulated magnesium-containing product which is commercially sold under the designation granulated potassium with MgO.

According to the invention, the dustlike particles 1 are separated and withdrawn in the first electrostatic separation device. The'middlings 2 containing K 0 are introduced into a second electrostatic separator wherein at the positive electrode a practically dust-free concentrate 4 will be separated. The middlings 5 of the second separation are preferably ground and then recycled for reintroduction into the first electrostatic separating device, together with additional preconditioned crude salt.

The residue 3'which accrues at the zones of the negative electrode in the first and second separating device, is withdrawn in conventional manner.

The process of the present invention thus provides for the separation of a dust concentrate 1 from the fraction of coarser salt particles 4.

By proceeding in this manner, and introducing tons per hour of crude salt into the illustrated electrostatic separator, it is possible to recover 2 tons per hour of a dust concentrate 1 containing 40.5 percent K 0 and about 5 tons per hour of a coarse preconcentrate 4 containing between 44 and 46 percent K O. The dust concentrate 1 contains the dustlike particles of sylvite having a size of less than 0.15 mm. and the coarse concentrate 4 contains the coarser sylvite particles having a particle size of 0.15-1.0 mm., which are then subjected to a second electrostatic separation in order to obtain a product of up to about 60 percent K 0 as described further above.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What we claim as new and desire to be secured by Letters Patent is:

1. A process'for the continuous electrostatic separation of ground potassium raw salts containing finely divided particles and being conditioned with a 10-15 percent aqueous solution of Na-salts of straight-chain, lower fatty acids comprising the steps of (at) establishing between successive areas of two endless traveling surfaces of opposite polarity a first electrostatic field having a strength of at least 3.5 and at most 4.5 kv./cm.; (b) feeding in a predetermine direction through said electrostatic field a stream of said potassium raw salts including particles having a grain size smaller than 0.15 mm. and particles having a grain size larger than 0.15 mm. and containing a predetermined percentage range of K 0 whereby a dust concentrate consisting of particles having a grain size up to about 0.15 mm. and containing between substantially 38-42 percent K 0 adheres to the positive one of said electrodes, while a dust-free potassium oxide-rich preconcentrate having a grain size greater than 0.15 mm. and containing 40-46 percent K O flows freely towards and past said positive electrode, and a potassium-poor residue flows to the negative one of said electrodes with a middling fraction passing intermediate both of said electrodes; (c) brushing said dust concentrate off and recovering it from said positive electrode; passing said middling fraction through a second similar electrostatic field between two additional electrodes of opposite polarity whereby an additional dust-free preconcentrate containing K 0 and having a grain size above about 0.15 mm. is separated from said middling fraction and freely passes over the positive additional electro e, while a residue poor in K 0 freely passes over the negative additional electrode with a residual dust-free fraction passing freely between both of said additional electrodes; and (d) recirculating said residual middling fraction to said first electrostatic field, together with new raw salts.

2. A process as defined in claim 1, wherein said stream is passed through said fields in downward direction.

3. A process as defined in'claim 1, wherein said raw salts are hartsalz.

4. A process as defined in claim 1, wherein the step of brushing said dust separately off said positive electrode comprises subjecting said positive electrode to a brushing with a stationary brush.

5. A process as defined in claim 1; and further comprising the steps of separately subjecting said additional preconcentrate to further separation into portions of higher and lower K 0 concentration until a portion is obtained having a K 0 concentration of between about 58 and 60 percent.

6. A process as defined in claim 1, wherein the steps of establishing said electrostatic fields comprise establishing at least said first electrostatic field at a field strength of between substantially 4.0 and 4.5 kv./cm.

7. A process as defined in claim 6, wherein the steps of establishing said electrostatic fields comprise establishing said second electrostatic field also at a field strength of between substantially 4.0 and 4.5 kv./cm.

8. A process for the continuous electrostatic separation, by means of continuous electrode surfaces of an endless, conductive, moving belt, of ground potassium raw salts containing finely divided particles and being conditioned with a 10-15 percent aqueous solution of sodium salts of straight chain, lower fatty acids, comprising the steps of establishing between two electrodes of opposite polarity in an electrostatic separator an electrostatic field having a strength of at least 3.5 and at most 4.5 kv./cm.; feeding in a predetermined direction through said electrostatic field a stream of said potassium raw salts including particles having a grain size smaller than 0.15 mm. and particles having a grain size larger than 0.15 mm. and containing a predetermined percentage range of K 0, whereby a dust concentrate consisting of particles having a grain size up to about 0.15 mm. and containing between substantially 38-42 percent K 0 adheres to the positive one of said electrodes, while a dust-free potassium oxide-rich precon centrate having a grain size greater than 0.15 mm. and containing 40-46 percent K 0 flows freely toward and past said positive electrode, and a potassium-poor residue flows to the negative one of said electrodes with a middling fraction passing intermediate both of said electrodes; brushing said dust concentrate off and recovering it from said positive electrode; and returning said middling fraction upstream of said electrostatic field for renewed passage therethrough with fresh raw salts.

9. A process as defined in claim 8, wherein the step of establishing said electrostatic field comprises establishing said field at a field strength of between substantially 4.0 and 4.5 kv./cm.

10. A process as defined in claim 9, wherein said stream is passed through said field in downward direction.

11. A process as defined in claim 9, wherein said raw salts are hartsalz.

12. A process as defined in claim 9; and further comprising the step of subjecting said preconcentrate to further electrostatic separation into portions of higher and lower K 0 concentration until a portion is obtained having a K 0 concentration of between about 58 and 60 percent. 

2. A process as defined in claim 1, wherein said stream is passed through said fields in downward direction.
 3. A process as defined in claim 1, wherein said raw salts are haRtsalz.
 4. A process as defined in claim 1, wherein the step of brushing said dust separately off said positive electrode comprises subjecting said positive electrode to a brushing with a stationary brush.
 5. A process as defined in claim 1; and further comprising the steps of separately subjecting said additional preconcentrate to further separation into portions of higher and lower K2O concentration until a portion is obtained having a K2O concentration of between about 58 and 60 percent.
 6. A process as defined in claim 1, wherein the steps of establishing said electrostatic fields comprise establishing at least said first electrostatic field at a field strength of between substantially 4.0 and 4.5 kv./cm.
 7. A process as defined in claim 6, wherein the steps of establishing said electrostatic fields comprise establishing said second electrostatic field also at a field strength of between substantially 4.0 and 4.5 kv./cm.
 8. A process for the continuous electrostatic separation, by means of continuous electrode surfaces of an endless, conductive, moving belt, of ground potassium raw salts containing finely divided particles and being conditioned with a 10-15 percent aqueous solution of sodium salts of straight chain, lower fatty acids, comprising the steps of establishing between two electrodes of opposite polarity in an electrostatic separator an electrostatic field having a strength of at least 3.5 and at most 4.5 kv./cm.; feeding in a predetermined direction through said electrostatic field a stream of said potassium raw salts including particles having a grain size smaller than 0.15 mm. and particles having a grain size larger than 0.15 mm. and containing a predetermined percentage range of K2O, whereby a dust concentrate consisting of particles having a grain size up to about 0.15 mm. and containing between substantially 38-42 percent K2O adheres to the positive one of said electrodes, while a dust-free potassium oxide-rich preconcentrate having a grain size greater than 0.15 mm. and containing 40-46 percent K2O flows freely toward and past said positive electrode, and a potassium-poor residue flows to the negative one of said electrodes with a middling fraction passing intermediate both of said electrodes; brushing said dust concentrate off and recovering it from said positive electrode; and returning said middling fraction upstream of said electrostatic field for renewed passage therethrough with fresh raw salts.
 9. A process as defined in claim 8, wherein the step of establishing said electrostatic field comprises establishing said field at a field strength of between substantially 4.0 and 4.5 kv./cm.
 10. A process as defined in claim 9, wherein said stream is passed through said field in downward direction.
 11. A process as defined in claim 9, wherein said raw salts are hartsalz.
 12. A process as defined in claim 9; and further comprising the step of subjecting said preconcentrate to further electrostatic separation into portions of higher and lower K2O concentration until a portion is obtained having a K2O concentration of between about 58 and 60 percent. 